[exim] Stopping relaying to internet hosts

Hi

I have a host running exim 4.43, which sends and receives mail OK. The
thing is, I don't want students sending email to remote hosts on the
internet. ie. I only want to be able to send mail to one host on our
network. Is it easier to firewall it, or do it more gracefully with
exim. I am new to exim 4.

Kev